NUMERICAL INVESTIGATION OF FLOW STRUCTURE AND PRESSURE DROP PREDICTION FOR RADIAL INFLOW BETWEEN CO-ROTATING DISCS WITH NEGATIVE EFFECTIVE INLET SWIRL RATIO

This paper presents a numerical simulation of the flow structure of radial inflow between co-rotating discs with a negative ceff (effective inlet swirl ratio), which may occur in a vortex reducer equipped with deswirl nozzles. When the value of ceff approaches zero, asymmetric flow structure is observed in the cavity, possibly as a result of the "Coanda effect". Besides this, the flow structure inside the disc cavity at ceff < 0 can be divided into a source region, a sink region, an interior core region, and two Ekman layers, which is identical to the situation when 0 < ceff ≤ 1. However, there exist two distinct patterns: the stagnation point on the disc and on the peripheral. According to a theoretical analysis, ceff = -1/8 is used to distinguish between these two patterns. Based on flow structure partitioning, a theoretical model for predicting the swirl ratio(V̄) radial distribution and pressure drop in a disc cavity with ceff < 0 was established. The model employs the turbulent boundary layer integral method, and von Karman's assumption of velocity profile and wall shear stress for a free disc. The calculation results of the swirl ratio in the cavity are in good agreement with the CFD results except when the negative ceff approaches zero because of the deviation of the radial velocity profile from the "1/7" power law. Since the pressure drop in the cavity approaches zero at the same time when the negative ceff approaches zero, the absolute error of the theoretical model in predicting pressure drop is not significant and does not affect its practical application, which has been verified through comparison with public experimental results.